1. Field of the Invention
The present invention relates to a photographic paper, and more particularly to a photographic paper coated with a polyolefin resin.
2. Description of Prior Arts
It is often experienced that a marking due to electrostatic charge (i.e., static mark) appears on a photosensitive surface of a photographic paper coated with a polyolefin resin. The electric charge is generated by friction caused between the surface of the polyolefin resin layer and surfaces of rollers in a process of the preparation of a photographic paper, as well as in a developing process. More specifically, in a process of the preparation of a photographic paper, a polyolefin resin-coated support is coated with a photographic emulsion and then transferred to a dryer part of the emulsion coater, a winder part, and a slitter. In the latter stages, electric charge is generated by friction between the surface of the polyolefin resin layer and the surfaces of the rollers of these parts. In the developing process, a photographic paper is caused to pass through rollers in an automatic printer, an automatic developing machine, etc. In these stages, electric charge is also generated in the same manner. When the charge is discharged, the photographic paper keeps thereron the mark of the discharge as a latent mark. The discharge marks are observed as uneven fogs on the surface of the photographic emulsion layer after the paper is developed.
For the above-described reason, various methods have been utilized in practice to reduce the generation of the electrostatic charge. Examples of the known methods include a method comprising incorporation of an antistatic agent into a polyolefin resin, a method comprising incorporation of an antistatic agent into a paper sheet, and a method of coating a hydrophilic colloid layer having antistatic activity over the back surface of polyolefin resin layer (i.e., a surface opposite to the photographic emulsion-coated surface).
However, these method have following drawbacks, and no satisfactory method has been known for the following reasons.
For the first method, almost no practically suitable antistatic agents are known, because an organic antistatic agent tends to decompose at the high temperature employed for the extrusiton of the polyolefin resin, and an inorganic antistatic agent is not suitable for a photographic paper due to the fact that it causes reduction of strength of the resin layer as well as reduction in whiteness.
Alternatively, there can be employed the third method providing a hydrophilic colloidal layer on the surface of the polyolefin resin on the back side. This method can be performed by coating the polyolefin layer with a hydrophilic polymer material such as gelatin, carboxymethylcellulose, hydroxyethylcellulose, polyvinyl alcohol, or polyacrylamide. However, this method also has various disadvantages, for instance, in the developing stage, the hydrophilic polymer material layer is easily separated or decomposed. Othewise, the hydrophilic polymer material layer tends to adhere to a surface of other photographic paper, or shows sticky property. Accordingly, this method is employed only under restricted conditions.
For the above reasons, the second method comprising addition of an antistatic agent such as an inorganic salt has been utilized more generally. This method is disclosed, for instance, in U.S. Pat. No. 3,253,922, British Pat. No. 1,346,960, and Japanese Patent Publications No. 50(1975)-3114 and No. 56(1981)-53744. However, in the second method, the addition of inorganic salt to a paper sheet causes the following troubles. If an inorganic salt is added in a large amount, pin holes are likely formed in the polyolefin resin layer when the polyolefin layer is subjected to corona discharge treatment. This treatment is usually applied to strengthen bonding between the polyolefin resin layer and the emulsion layer. Further, the photographic characteristics tends to deteriorate during storage after the application of the photographic emulsion. In more detail, fogs likely occur on the photographic emulsion layer.
In addition, the inorganic salt antistatic agent possibly causes rusting in a processing stage such as a surface surface size stage. In view of the possible troubles, the inorganic salt agent should be used in an amount as small as possible. Thus, the inorganic salt antistatic agent is preferably added in an amount of 2% or less based on the weight of paper sheet so that these troubles can be obviated. However, the addition of inorganic salt antistatic agent in the above range exhibits only insufficient antistatic property. Therefore, the processing rate for photographic paper is disadvantageously restricted.